A super heterodyne transmitter is a type of transmitter that uses an intermediate frequency (IF) in addition to a radio frequency (RF). Within the transmission chain of a super heterodyne transmitter, a mixer converts the IF signal to an RF signal by mixing the IF signal with a signal generated by the local oscillator (LO). In an ideal system, the IF signal and (LO) signal are mixed perfectly to generate an RF signal at the desired frequency range. However, in real-word applications, LO leakage causes unwanted additions to the RF signal. Making matters more complex, LO leakage normally varies over frequency, temperature, gain, and power level and can therefore not easily be calibrated out.
One approach for reducing leakage is to use a band stop filter to prevent the LO frequency component from reaching the transmitter power amplifier. This approach limits the frequency range that the transmitter can cover without changing filters and adds complexity to the system. It also prevents the transmitter chain from being modulated directly at baseband in applications where this is a requirement.
Another approach is to use a quadrature type upconverter to apply fixed DC offsets at complex baseband. The upconverter converts the fixed DC offsets into a cancellation vector, which is added to the baseband signal to cancel the noise introduced by the LO leakage. This approach is effective when there is little variability in LO leakage; however, it adds calibration complexity and hardware stability requirements, resulting in higher cost and lower performance.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.